Chapter 29: Metamorphism of Calcareous and Ultramafic Rocks. Calcareous rocks are predominantly carbonate rocks, usually limestone or dolostone Typically form in a stable continental shelf environment along a passive margin

Copyright Complaint Adult Content Flag as Inappropriate

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

Figure 29.1.Chemographics in the CaO-MgO-SiO2 -CO2 -H2O system, projected from CO2 and H2O. The green shaded areas represent the common composition range of limestones and dolostones. Due to the solvus between calcite and dolomite, both minerals can coexist in carbonate rocks. The dark red left half of the triangle is the area of interest for metacarbonates. Carbonated ultramafics occupy the right half of the triangle. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 29.3.T-XCO2 phase diagram for siliceous carbonates at P = 0.1 GPa. Calculated using the program TWQ of Berman (1988, 1990, 1991). The green area is the field in which tremolite is stable, the reddish area is the field in which dolomite + diopside is stable, and the blue area is for dolomite + talc. Compatibility diagrams, similar to those in Figure 29.4, show the mineral assemblages in each field. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 29.4.The sequence of CaO-MgO-SiO2-H2O-CO2 compatibility diagrams for metamorphosed siliceous carbonates (shaded half) along an open-system (vertical) path up metamorphic grade for XCO2 < 0.63 in Figure 29.3. The dashed isograd requires that tremolite is more abundant than either calcite or quartz, which is rare in siliceous carbonates. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.

Figure 29.6.T-XCO2 phase diagram for siliceous carbonates at P = 0.5 GPa, calculated using the program TWQ of Berman (1988, 1990, 1991). The light-shaded area is the field in which tremolite is stable, the darker shaded areas are the fields in which talc or diopside are stable. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 29.7a.T-XH2O diagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the Whetstone Lake area. Reactions 1, 2, and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in metapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a temperature maximum at XH2O = 0.25. After Carmichael (1970) J. Petrol., 11, 147-181.

Figure 29.7b.Isograds mapped in the field. Note that isograd (5) crosses the others in a manner similar to that in part (a). This behavior is attributed to infiltration of H2O from the syn-metamorphic pluton in the area, creating a gradient in XH2O across the area at a high angle to the regional temperature gradient, equivalent to the T-X diagram. After Carmichael (1970) J. Petrol., 11, 147-181.

Figure 29.7a.T-XH2O diagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the Whetstone Lake area. Reactions 1, 2, and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in metapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a temperature maximum at XH2O = 0.25.b.Isograds mapped in the field. Note that isograd (5) crosses the others in a manner similar to that in part (a). This behavior is attributed to infiltration of H2Ofrom the syn-metamorphic pluton in the area, creating a gradient in XH2O across the area at a high angle to the regional temperature gradient, equivalent to the T-X diagram. After Carmichael (1970) J. Petrol., 11, 147-181.

Figure 29.8.Schematic T-XCO2 diagram illustrating the characteristic shape of typical dehydration reactions, such as those that generate orthopyroxene from hornblende or biotite. Notice that the amphibolite facies to granulite facies can be accomplished by either an increase in temperature or infiltration of CO2 at a constant temperature. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Alpine peridotites: uppermost mantle = base of slivers of oceanic lithosphere that become incorporated into the continental crust along subduction zones

Dismembered portions of ophiolites: pieces of oceanic crust and mantle that either separate from the subducting slab and become incorporated into the accretionary wedge of the subduction zone, or (more commonly) get trapped between two terranes during an accretion event

Chain of ultramafic bodies in Vermont indicating a suture zone of the Ordovician Taconic Orogeny. The ultramafics mark a closed oceanic basin between North American rocks and an accreted island arc terrane. From Chidester, (1968) in Zen et al., Studies in Appalachian Geology, Northern and Maritime. Wiley Interscience.

Figure 29.11.Chemographics of ultramafic rocks in the CMS-H system (projected from H2O) showing the stable mineral assemblages (in the presence of excess H2O) and changes in topology due to reactions along the medium P/T metamorphic field gradient illustrated in Figure 29.10.Star represents composition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO2-rich or SiO2-poor varieties. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monogr. 1.